In the art of precision cleaning, it is necessary to remove liquids and solids from surfaces of workpieces leaving high purity surfaces. Precision cleaning involves the use of solvents and often more than one solvent. Since the workpiece surface can be no cleaner than the cleanliness of the solvents used to clean it, it is necessary to flush solvents through the cleaning chamber in precision cleaning methods.
As a practical matter in precision cleaning methods, solvents must be recycled. Their initial cost is not insignificant nor is the cost of disposing contaminated solvents.
It is often the case that more than one type of solvent is required for precision cleaning because the solvents tend to be selective of the liquids and solids taken into solution. Cleaning with mixed cosolvents simultaneously applied to the workpiece has been considered. This results in the major contamination of one solvent with another making the recycling of either solvent more complicated.
A cleaning fluid comprising liquid or supercritical carbon dioxide has been found to be an excellent solvent because of its ability to take into solution a range of solid and liquid contaminants and because it can be recycled in a separation stage during which it is depressurized to permit it to vaporize. However, no vapor separation process is perfect and the product of the separation depends upon the degree and type of contamination.
Some very small particles cannot be dislodged from the workpiece surface unless the solvent in contact therewith is agitated as by ultrasound applied to the solvent and for very small particles unless agitation as by ultrasound vibrations is directly applied to the workpiece. Some liquid solvents in contact with certain piezoelectric materials cause them to degrade. Hence, it is desirable to isolate the piezoelectric transducers from those liquid solvents.
It is an advantage, according to this invention, to provide precision cleaning of a workpiece or workpieces in which the liquid and/or supercritical fluid carbon dioxide is used in combination with another solvent and/or other solvents but without major intermixing of solvents that is a drawback to recycling of the carbon dioxide.
It is a further advantage, according to this invention, to provide precision cleaning of a workpiece or workpieces in which liquid and/or supercritical fluid carbon dioxide is used in combination with another solvent or other solvents and ultrasound can be applied to either the solvents or workpiece without damaging the piezoelectric materials.
It is yet another advantage, according to this invention, to provide precision cleaning with a liquid solvent followed by the cleaning with a cleaning fluid comprising liquid or supercritical fluid carbon dioxide while minimizing the intermixing of the liquid solvent and the cleaning fluid and thereby eliminating the disadvantages therewith.